fix(mermaid): make the download control reachable in the Mermaid fullscreen view

[dkachur1]

Summary

When a Mermaid diagram is expanded to fullscreen, there is no way to download it — the download button disappears entirely.

Root cause

MermaidFullscreenButton renders its overlay via createPortal(..., document.body). The block's action toolbar (MermaidDownloadDropdown, copy button, MermaidFullscreenButton itself) is rendered as a sibling of the diagram inside components.tsx, outside that portal. Since the portal is fixed inset-0 z-50, it visually covers that toolbar, and structurally the toolbar was never included in the portal's own markup — so once fullscreen is open there's no download affordance reachable at all.

This is the same class of bug already fixed for tables in #470 (fix(table): add table-wrapper to fullscreen portal so copy/download work), closed by #468 — except for tables the buttons were present in the portal but a closest() DOM lookup failed; for Mermaid the fullscreen view never rendered a download control in the first place.

Fix

Render MermaidDownloadDropdown inside the fullscreen portal, next to the exit button, respecting the existing controls.mermaid.download config (mirrors the same inline boolean-resolution pattern already used for panZoom in this file). Propagation is stopped on the wrapping div so clicking download doesn't also trigger the backdrop's close-on-click handler.

Testing

• Added two tests to mermaid-fullscreen.test.tsx:
  • the download button renders inside the fullscreen portal, downloads a PNG on click, and does not close the fullscreen overlay
  • the button is hidden when controls.mermaid.download is false
• All existing tests in mermaid.test.tsx, mermaid-component.test.tsx, mermaid-download.test.tsx, mermaid-fullscreen.test.tsx pass (56/56).
• biome check clean on both changed files.

Added a changeset.

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